1. Field of the Invention
The present invention relates to an optical device for correcting aberrations affecting an image. In particular, a device according to the invention enables the distortion due to a spherical concave mirror inclined with respect to the direction from which this mirror is observed to be corrected.
The invention is applicable especially, but not exclusively, to a helmet sight for the pilot of combat aircraft or helicopters or for the operator of a training simulator.
2. Discussion of the Background
A helmet sight is an image presentation device integrated into a helmet. The sight enables the person wearing the helmet, for example the pilot of an aircraft in flight, to observe visual information at the same time as viewing the scenery, or the cockpit, which he perceives most often through a protective visor.
The presentation of appropriate information, for example in the form of symbols, provides assistance to piloting and navigation. So, for armed vehicles, the presentation of a reticule provides assistance when aiming a weapon.
The information may also consist of an image of the scenery acquired by sensors other than the eyes of the helmet wearer, such as infrared image sensors or light intensifiers to complement or replace direct vision.
Inside the helmet, an image generator comprises an imager whose screen, for example a cathode-ray tube screen or a liquid-crystal screen, enables an image to be displayed.
The image is most often conveyed using an optical relay system up to a combiner which ensures that the conveyed image is presented superimposed on the view of the scenery.
To enable the pilot to simultaneously observe the scenery viewed directly at infinity and the image from the imager, the image is also being focused at infinity by an optical collimation system.
When the combiner is formed from a simple semi-reflective flat plate, the image may be collimated by an optical system placed between the imager and the combiner; such an embodiment of the prior art has the main drawback of needing a optical collimation system which is too bulky with respect to the restricted field of view obtained.
To reduce the bulk, a combiner with optical power has been proposed; such a combiner provides its user with both the collimation of the image and the superposition of the collimated image with the view of the scenery.
The prior art has a wealth of many and varied devices comprising a combiner with optical power. Of particular interest are the image presentation systems comprising a spherical concave mirror to collimate the image.
A spherical concave mirror provides an average quality collimation of an image placed at a particular point in the space located on the axis of the mirror and at a distance from the latter equal to half its radius of curvature. By placing an imager at this point, the eye located on the axis of a mirror receives rays coming from the imager after their reflection from the spherical mirror; these rays are parallel and lead to the perception by the eye of a collimated image. If, furthermore, the mirror is semi-reflective, it enables the same eye to observe the scenery in transmission. However in such a device the imager would have to lie on the axis of the semi-transparent spherical mirror and it would block the user""s field of view.
To clear the user""s view, the spherical mirror is inclined with respect to the normal to the face and the user""s eye is no longer on the mirror axis. This arrangement has the drawback of leading to a collimated image affected with optical aberrations, especially off-centring aberrations, which need to be corrected, at least partially.
The inclination of the spherical concave mirror impairs the collimated image with distortion, called off-centring distortion of the second kind, characterized by a convergence of the verticals and an apparent curvature of the horizontals.
The prior art teaches us to correct the distortion of the image provided by an optical assembly by introducing an inverse distortion in the imager by electronic correction; this is easily achieved when the imager has a cathode-ray tube but this solution is not suitable for an imager, for example a light intensifier, which does not have the required means for adjusting the image. It might also be possible to try to correct the distortion by inserting another inclined spherical mirror into the optical path between the imager and the spherical mirror, introducing a distortion which is the inverse of the first; however this would lead to an optical system which is unusable because of its bulk.
The problem consists in producing an imaging device comprising a spherical collimation mirror, having a collimated image which satisfies the user, i.e. an image free from annoying aberrations and having a wide field of view, preferably greater than or equal to 40 degrees. The object is to obtain a collimated image which has both a high resolution and good correction of the distortion. The distortion to be corrected, due to a spherical collimation mirror observed at an oblique angle with respect to the axis of the mirror, is an off-centred distortion of the second kind. The difficulty consists in finding a means to correct the distortion without degrading the image quality, while at the same time having a low mass, bulk and cost and which is easy to manufacture.
For this reason, the invention proposes an optical device for a system presenting collimated images to a user, comprising an imager and an off-axis spherical mirror, characterized in that it comprises means to correct the distortion of the image presented to the user which is due to the spherical mirror, the said means comprising a tubular concave mirror located between the imager and the spherical mirror.
The term tubular mirror refers to a mirror whose surface is generated by the displacement of a portion of a first plane curve in translation along a portion of a second curve. The first plane curve is preferably in a plane perpendicular to the plane of symmetry of the system. The second curve is also preferably plane and located in the plane of symmetry of the image presentation system. The portion of the first plane curve is preferably a circular arc and the portion of the second curve as well, but it might be possible to envisage these curves being conic sections (ellipse, parabola, hyperbola).
The surface of the tubular concave mirror provides assistance in correcting the distortion of the image presented to the user by an off-axis spherical collimation mirror, and, furthermore, this type of mirror is particularly easy to machine, especially if the first curve (that which undergoes translation to generate the surface of the mirror) is a circular arc.
The device also has one or more optical power or relay groups placed on the ray path between the imager and the spherical mirror, upstream and/or downstream from the tubular mirror.
The correction of the distortion by a tubular mirror is in principle considerably better since these optical groups give the beams coming from the imager and incident on the tubular mirror a smaller aperture (while a large beam aperture is desirable at the spherical mirror).
These optical groups also ensure precorrection of the astigmatism necessarily introduced into the collimated image because the spherical mirror is observed at an angle inclined with respect to the radius which defines the optical axis of this mirror. This astigmatism may be corrected, for example, by a spherical convergent lens and a cylindrical lens, in an optical relay group located between the imager and the tubular mirror. It may also be corrected by a diffractive lens placed in a power group between the tubular mirror and the spherical mirror.
In one particular embodiment, a power group may be provided between the tubular mirror and the spherical mirror with the following particular features: it has a convergent lens whose focus is virtually centred on the first pupil image which is the image of the pupil of the eye formed by the off-axis spherical mirror. The power group focuses the beams of optical rays onto the tubular mirror, which beams, coming from the pupil of the eye, have been reflected by the inclined spherical collimation mirror. These beams are almost parallel.
The invention enables a high resolution image to be maintained while ensuring substantial correction of the distortion due to the inclined spherical collimation mirror. The invention has the advantage of correcting the distortion of the image presented to the user""s eye by a wide instrument pupil, for example one of at least 15 millimetres diameter, and for a wide field typically greater than 40 degrees. The instrument pupil is the region of space in which the user of an instrument must place the pupil of his eye in order to use it.
This correction is particularly beneficial when a distortion cannot be readily imposed on the imager. This is because an electronic correction of the prior art is not suitable in such a case.
The first pupil image of the device is inclined with respect to the optical axis, the tubular concave mirror according to the invention gives a second pupil image of it which is rectified on the optical axis.
The invention may be integrated into a helmet sight having a wide instrument pupil and a wide field.